In the early form of the thermopile type radiation pyrometer the target area upon which the radiation was directed and which contained the hot junctions of the thermopile was spatially supported by the thermocouple wires in order to reduce to a minimum the thermal conductivity from the hot junctions on the target area to the cold junctions associated with the body or case of the radiation pyrometer. More recently, attention has been paid to an arrangement referred to as a solid-backed thermopile in which a thin insulating layer extends across an opening in a support frame and in which the thermopile is formed by depositing on the insulating layer a series of thermocouple junctions with the hot junctions located in an area in the center of a central opening in the frame and the cold junctions located in an area corresponding with the frame material. U.S. Pat. Nos. 3,424,624 issued to P. Villers et al and 3,405,273 issued to N. B. Stevens et al disclose two different approaches to the solid-backed thermopile. The common material, as shown in these two patents for the thin insulating film, has been either Mylar film or a film of aluminum oxide. In constructing thermopiles of this type it has been the practice to attach the supporting film by means of an adhesive to the supporting frame although aluminum oxide films have been prepared by anodizing a block of aluminum and etching away the central portion of aluminum to form an unsupported film of aluminum oxide. In order to obtain the maximum output from the thermopile for the number of thermopile junctions, the material generally used in this type of device are bismuth and antimony because of their large Seebeck coefficients. U.S. Pat. No. 3,424,624 discusses the construction of a thermopile indicating that the radial thermopile is the conventional arrangement although thermopiles may be arranged with the junctions in rows such as shown in U.S. Pat. No. 3,405,273.
The performance of a thermopile radiation detector is in general measured by the voltage output from the thermopile per watt of radiant energy received and also the time constant of response for the thermopile. Quite obviously, a low-time constant and a large voltage output per watt of received energy are the objectives in any design. In a solid-backed thermopile this performance is, to large measure, determined by the number of thermocouple junctions that can be located in the active target area and the amount of conductivity of heat from the hot junctions to the cold junctions through the insulating layer, as well as the more conventional problem of dissipating heat from the cold junction region.